The subject matter of this invention is related to the subject matters of inventions entitled "Switch Components and Multiple Data Rate Non-Blocking Switch Network Utilizing the Same" U.S. Ser. No. 07/283,173 issued as U.S. Pat. No. 4,914,429, "System for Cross Connecting High Speed Digital Signals" U.S. Ser. No. 07/283,171, and "System for Cross Connecting High Speed Digital SONET Signals" U.S. Ser. No. 07/283,172, all of which are filed of even date, are of common inventorship, are assigned to the assignee hereof, and all of which are hereby incorporated by reference herein.
This invention relates generally to switch components and non-blocking switching networks utilizing a plurality of switch components. More particularly, the invention relates to a switching component capable of receiving data from at least one virtual tributary of a larger signal, retiming the data, and switching that data at a desired time to a desired location. The switching network utilizing a plurality of such switch components receives a plurality of virtual tributaries, many of the virtual tributaries constituting a SONET signal, and creates new SONET signals comprised of rearranged virtual tributaries.
Standards for SONET telecommunication have been promulgated by ANSI in ANSI T1.105-1988 and are detailed in SONET Transport Systems, Common Generic Criteria, TA-TSY-00253 Issue 3, July 1988, published by Bellcore, as well as other documents known to those skilled in the art. A basic SONET signal, termed an STS-1 signal, is seen in FIG. 1. The SONET signal is a 51.84 Mhz, bit-serial signal, having nine rows of ninety columns of eight bit bytes at a 125 microsecond frame rate. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes and are used for various control purposes as indicated in FIG. 2. The remaining eighty-seven columns of bytes constitute the STS-1 Synchronous Payload Envelope (SPE) as seen in FIG. 3.
Turning to FIG. 2, it is seen that the first two bytes A1 and A2 of the transport overhead are framing bytes which contain a specified framing pattern allowing synchronization of the basic SONET STS-1 signal. Three other bytes, H1, H2, and H3 form a pointer giving explicit information as to the location of the start of the SONET SPE. The pointer bytes are required due to the fact that the position of the SPE is not fixed in time in the STS-1 frame, but is allowed to be displaced in time. The SPE may move slowly backward and forward in time relative to the STS-1 frame due to varying circuit conditions. Hence, as seen in FIG. 3, a single SPE is seen to typically straddle two consecutive STS-1 frames.
In actuality, as seen in FIG. 4 which sets forth the payload pointer coding, the pointer for the SPE is located in the last ten bits of the word formed by bytes H1 and H2. The pointer value is an offset value and designates the location after byte H3 of the first byte of the SPE. Thus, if the pointer value is zero (i.e. zero offset), the first byte of the SPE is located in the first byte position after the H3 byte. If the pointer value equals one, the SPE starts at the second byte past byte H3. The greatest value allowed for the pointer is seven hundred eighty-two (equal to 810 - 27 - 1; 810 bytes in the frame, less 27 bytes for the TOH, less one byte to find the final location). The value of seven hundred eighty-two indicates, as seen in FIG. 5 which shows byte locations, that the SPE starts at the last byte position before the H1 byte of the next STS-1 frame.
As indicated in FIGS. 4 and 5, during normal operation, two kinds of pointer adjustments are allowed. A negative stuff is utilized when the SPE being transported is running at a frequency higher than that of the STS-1 envelope (i.e. additional information must be inserted into the envelope), while a positive stuff is utilized where the SPE is running at a frequency slower than the STS-1 signal (i.e. stuff bytes are inserted into the envelope). Regardless, the SPE phase is moved by one byte, forward or backward in time.
Turning to FIG. 6, it is seen that the payload of the SONET signal is subdivided into a number of "virtual tributaries" (VTs), where a virtual tributary is an arrangement of a specified number of bytes in the SPE payload. Column one of the STS-1 SPE contains control information termed "path overhead" (POH) which is relevant to the VTs of the payload, while columns thirty and fifty-nine contain fixed stuff which is essentially irrelevant for purposes herein. SONET signals are composed of a number of VTs which may all be of the same, or an allowed mix of sizes; e.g. VT1.5, VT2, VT3, VT6. In the United States, a SONET signal is typically comprised of twenty-eight DS-1 virtual tributaries. For example, a DS-1 signal is carried on a VT1.5 which consists of three columns of bytes in designated time locations. In FIG. 6, virtual tributary number two is shown in columns three, thirty-two and sixty-one of the eighty-seven column SPE.
As seen in FIGS. 2 and 6, the path overhead includes a byte H4 which is termed the "multiframe indicator". The multiframe indicator is used to establish which of four bytes V1, V2, V3 and V4 which index four phases of a four-frame SPE superframe cycle is in the first location of the currently-received payload. Coding of the VT pointers is seen in FIG. 8, with V1 and V2, which are located in two consecutive STS-1 SPE's, containing a pointer to the virtual tributary synchronous payload envelope (which extends over four SONET frames; hence the need for phase indexing). The start of te VT-SPE is the V5 byte. The byte offset definitions, and hence the V5 byte positions are shown in FIG. 9. As indicated in FIG. 9, positive and negative stuffs are allowed at the VT level, much as it is at the STS-1 level, with a positive stuff required when the signal carried in the VT is running slow, and a negative stuff required when it is running fast.
SONET signals are allowed to be of various modes, including asynchronous, bit synchronous floating or locked, and byte synchronous floating or locked. The switching network performs the required cross-connect functions on asynchronous VTs and on bit or byte synchronous VTs of the floating mode. To cross-connect locked mode VTs, they must first be converted to floating mode in a manner such as described in the BellCore TA-TSX-000253 publication, as their construction is different and contains no VT pointer.
The source of a SONET STS-1 signal is typically a multiplexer which can multiplex twenty-eight T-1 signals into twenty-eight VT1.5 virtual tributaries in a SONET formatted signal. In a system of multiple SONET links such as is seen in FIG. 10, it is often desirable to form a logical path between some T-1 end point X and another T-1 end point Y, where X and Y are on two different SONET STS-1 links P and Q. The system used to form such a logical path is called a cross-connect system. In particular, where SONET formatted signals are being cross-connected, the basic function of the cross-connect system is to switch the virtual tributaries of the incoming SONET signals in time and space into virtual tributaries of outgoing SONET signals. Special considerations in implementing such a system are the asynchrony of the input SONET signals and of their constituent VTs, and the need to reformat a number of mutually asynchronous VTs into a properly constructed outgoing SONET signal.